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The sightline to the brighter member of the gravitationally lensed quasar pair UM 673A,B intersects a damped Lyα system (DLA) at z= 1.62650 which, because of its low redshift, has not been recognized before. Our high-quality echelle spectra of the pair, obtained with HIRES on the Keck I telescope, show a drop in neutral hydrogen column density N(H i) by a factor of at least 400 between UM 673A and UM 673B, indicating that the DLA’s extent in this direction is much less than the 2.7 h−170 kpc separation between the two sightlines at z= 1.62650. By re-assessing this new case together with...

The sightline to the brighter member of the gravitationally lensed quasar pair UM 673A,B intersects a damped Lyα system (DLA) at z= 1.62650 which, because of its low redshift, has not been recognized before. Our high-quality echelle spectra of the pair, obtained with HIRES on the Keck I telescope, show a drop in neutral hydrogen column density N(H i) by a factor of at least 400 between UM 673A and UM 673B, indicating that the DLA’s extent in this direction is much less than the 2.7 h−170 kpc separation between the two sightlines at z= 1.62650. By re-assessing this new case together with published data on other quasar pairs, we conclude that the typical size (radius) of DLAs at these redshifts is R≃ (5 ± 3) h−170 kpc, smaller than previously realized. Highly ionized gas associated with the DLA is more extended, as we find only small differences in the C iv absorption profiles between the two sightlines.

Coincident with UM 673B, we detect a weak and narrow Lyα emission line which we attribute to star formation activity at a rate SFR ≳ 0.2 M⊙ yr−1. The DLA in UM 673A is metal poor, with an overall metallicity ZDLA≃ 1/30 Z⊙, and has a very low internal velocity dispersion. It exhibits some apparent peculiarities in its detailed chemical composition, with the elements Ti, Ni and Zn being deficient relative to Fe by factors of 2–3. The [Zn/Fe] ratio is lower than those measured in any other DLA or Galactic halo star, presumably reflecting somewhat unusual previous enrichment by stellar nucleosynthesis. We discuss the implications of these results for the nature of the galaxy hosting the DLA.